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Foation of submne gas hydrates
//
/
757
/
5
& G.
)
Fig. 1. Gas hydrate locations
in the Ocean. Gas hydrate
observation: 1 - deep sea
drilling, 2 - bottom
sampling; geochemical
indications: 3 - low
chlorinity of pore water, 4 -
gassy cores; geophysical
indications: 5 - logging data;
6 - seismic survey data.
forming gas. The low dissociation pressure of freon-12
hydrate in comparison with hydrate of natural gas pro-vided better conditions for the experiment. Moreover,
like hydrocarbon gases, freon-12 hydrate is hydrophobic.
Fig. 2 shows the arrangement of the experimental
equipment. The vessel for water saturation with gas (1)
and the reaction vessel (2) are the main items. The experi-
mental conditions were as follows: P = 200 kPa; Tl =
284.9 °K; PI = 432 kPa; T2 = 274.3 °K; P2 = 46.5 kPa
where P is the gas pressure in the plant during the time of
water saturation with gas and the beginning of gas hy-
drate formation. Tl is the temperature in the saturation
vessel; PI is the equilibrium pressure of freon-12 hydrate
formation at Tl; T2 is the temperature in reaction vessel
and P2 is the equilibrium pressure of freon-12 hydrate
formation at T2.
As a result of the experiment the freon-12 hydrate was
obtained in the reaction vessel directly from aqueous
solution. Gas hydrate formation was observed visually
and pressure in the plant was decreased from 200 to 170
kPa (Fig. 3). Oversaturation of the water with gas in the
reaction vessel was 3.4 at P = 200 kPa at the beginning of
gas hydrate formation and 2.9 at P = 170 pKa at the end
of the experiment.
Fig. 2. Scheme of the experimental set. 1 - vessel for water
saturation with gas, 2 - buffer volume for gas separation, 3 -
reaction vessel, 4 - refrigerator-thermostat, 5 - filter, 6 - glass
finger,
7
- gas-collector,
8
- magnetic
pump,
9 - pressure gauge,
10
- magnetic mixer,
11
- valves,
12
- resistance thermometer,
13 - system for gas pumping, 14 - system for filling with
solution; A - liquid phase; B - gaseous phase.
Soloviev & Ginsburg: Formation of submarine gas hydrates 87
160 10 20 30 40 30 ?0 SO
fO 20 30 40 50 60 70 80
Fig.
3.
Plot of pressure
(P,
kPa) and difference between tempera-
ture of thermostat and temperature behind reaction vessel (AT,
°C) vs. gas hydrate formation time (hours)
Observational data
The Caspian Sea
Specific features of the southern Caspian Sea are the
thick Cenozoic sedimentary cover, high oil and gas po-
tential and the wide distribution of clay diapirism and
mud volcanism. More than fifty mud volcanoes are
thought to occur in deep-water areas of the sea (Fig. 4)
Fig. 4. Gas hydrate accumulations in the deep southern Caspian
Sea. 1 - detected gas hydrate accumulations at the mud volca-
noes,
2 - submarine mud volcanoes, 3 - limit of the potential
gas hydrate prone area.
and gas hydrate accumulations associated with these mud
volcanoes have been discovered (Efremova et al. 1979;
Ginsburg et al. 1988). Diapiric structures and mud volca-
noes are clearly displayed on medium frequency seismic
records (Fig. 5).
Gas hydrates were observed in two mud volcanic crater
fields in a clay breccia, one occurrence being immedi-
ately on the sea bottom. The hydrate inclusions in the
breccia were up to 5 cm and of various shapes of which
the most common were subisometric, though sometimes
thin plates were observed. The hydrate content in the
Fig. 5. Seismic profile across
the Vezirov Rise (Shatsky
Ridge).
Arrows indicate
sampling stations; the left
scale is two-way traveltime
(s).
Bulletin of the Geological Society of Denmark
Table 1. Composition of gas from the gas hydrates.
Region
Caspian
Sea
Black
Sea
Okhotsk
Sea
Area
Buzdag, mud
volcano
Elm, mud
volcano
offshore
the Crimea
offshore Para-
mushir island
offshore
Sakhalin
island
n.a. = not analyzed
n.d. = not detected
SC+co2
%
99.0
93.4
99.5
43.0
98.2
99.2
74.0
97.9
96.2
100.0
Composition of gas
C,
74.7
76.0
95.3
81.4
99.1
99.1
99.9
99.9
99.3
97.0
c2
17.4
19.3
0.6
15.3
0.02
0.04
<0.03
0.05
<0.03
<0.03
C3
2.4
2.4
1.6
1.6
2-10-4
4-10-4
n.a.
n.a.
n.a.
n.a.
, % from 2(C
iC4
0.4
0.6
1.7
0.2
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
nC4
1.1
0.3
n.d.
0.7
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
„
+ C02)
C5+
0.33
0.05
0.01
n.d.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
co2
3.6
1.2
0.9
0.8
0.9
0.9
0.07
0.09
0.63
2.96
C,
-44.8
n.a.
-56.0
n.a.
-61.8
-63.4
-67.3
n.a.
-64.3
-59.6
-61.6
813C,
C2
-26.0
n.a.
-27.0
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
%c
C3
-22.1
n.a.
-7.8
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
c4
-20.9
n.a.
-30.8
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
breccias was up to 35 vol%. This high content may be
attributed to the contrasting rates of ascending hydrate-
forming fluids and mineral mass. Fluids ascend more
rapidly than volcanic mud. Hydrates present at the sea
bottom occur by the same process.
The gas from hydrate samples contains as much as
19.3%
ethane and 2.4% propane (Table 1). The high
content of methane homologues and the heavy isotopic
composition of carbon in the methane-ethane-propane-
butane series (813C values for the sample from Buzdag
are -44.8%c, -26.0%o,
-22.1%0
and -20.9%o, respec-
tively) attest to the catagenic nature of the gas. Gas
hydrates in the clay breccia coexist with very salty pore
water. The chlorinity of
the
interstitial water
is
locally ten
times as much as that of the Caspian bottom water (i.e.
5.6 g/1). Hence, gas hydrates are formed from mud vol-
l 0 V
E A /
—* '--o
:>::::x
*\
•. •
*
••
••••• 4
K~N. 1
**>•
M^
Fig.
6.
Gas hydrates and diapirs in the Black
Sea.
1
-
boundary
of
diapir distribution
zones,
2
-
boundary
of
submarine deltas and
fan
deltas,
3 - study area (gas hydrate locations in the Feodosia region).
Soloviev & Ginsburg: Formation of submarine gas hydrates 89
canic brine and the accumulation of gas hydrates is asso-
ciated with mud volcanoes and depends on the move-
ments of fluids.
The Black Sea
The gas hydrates in the Black Sea were discovered on the
lower part of the Crimean continental slope at a depth of
2050 m (Ginsburg et al. 1990)(Fig. 6) where gas hydrate
accumulation is associated with diapiric structures and
mud volcanoes (Fig. 7). The hydrates were found both in
clay breccia, where they formed inclusions ranging from
dispersed to massive, and in deformed silt-clay sediments
where the hydrates were observed as thin plates.
Methane is the main component of the Black Sea
hydrates (see Table 1). The water responsible for gas
hydrate formation is of a lower salinity than sea water. Its
composition is similar to that of the mud volcanic waters
in the Kerch-Taman region. The
8I3C
values of methane
from gas hydrates measured in two samples are -61.8%o
and -63.4%o which is typical of mud volcanic gases from
the same region. Hence, the hydrate-forming fluids
off-
shore from the Crimea must have come from the deep
sedimentary cover and the gas hydrate formation is due to
infiltration.
The Okhotsk Sea
Gas hydrate accumulations connected with submarine
gas seepage plumes were discovered and investigated
over two areas of the Okhotsk Sea (Ginsburg et al. 1992)
(Fig. 8). The locations of submarine seepages are easily
detected using echo sounding (Fig. 9) and coincide with
fracture zones. Eleven gas plumes were identified in the
Okhotsk Sea, one plume near Paramushir Island at a
depth about 800 m and ten plumes on the continental
slope off Sakhalin Island at depths of 620 to 1040 m. Gas
hydrate-bearing sediments were recovered in both re-
gions at subbottom depths of 0.3-1.2 m.
The hydrate cores obtained near Sakhalin Island
showed subhorizontal lenticular-bedded structure caused
by hydrates. The water content in the hydrate-bearing
sediments ranged from 65 to 66% in contrast to 48% to
60%
in the overlying sediments. We consider that the
subhorizontal structure and specific water distribution
near the upper boundary of the hydrate-bearing sediments
are brought about by the formation of hydrates from
upward diffusing methane whilst water moved in the
opposite direction to the front of the reaction.
All the gas hydrate occurrences in the Okhotsk Sea are
associated with carbonate cementation of the sediments.
Many carbonate concretions were observed together with
hydrates and allochthonous calcium carbonate deposits
were noted on mollusc shells. These features are quite
normal under gas seepage conditions. The carbonate con-
cretions are the result of methane oxidation which sat-
urates the pore water with carbon dioxide; calcium car-
bonate is then precipitated (Zonenshain et al. 1987).
Fig. 7. Seismic profile across diapirs in the Black Sea, Feodosia
region. Dotted lines are diapir limits; figures on the vertical
scales are two-way traveltime (s).
According to the data from deep seismic and conti-
nuous seismic profiling, the area of fluid discharge near
Paramushir Island coincides with a high of the acoustic
basement. The loss of definition and the pull-down of
reflectors are presumably attributable to low seismic ve-
locity in this part of the sedimentary sequence. This is
most probably associated with the occurrence of gas-
charged sediments above the acoustic basement. The sub-
marine fluid seepage fields off Sakhalin Island are in the
zone in which there are numerous submeridianal faults
along the west side of the Deriugin Basin near the oil and
gas areas of Sakhalin Island and the adjacent
shelf.
These
faults are most likely conduits for migrating gas as is
90 Bulletin of the Geological Society of Denmark
Fig. 8. Study areas in the
Okhotsk Sea.
Paramushir
Island
Japan
Fig. 9. Echo sounding
anomalies associated with
submarine gas discharge and
gas hydrate locations. A -
offshore Sakhalin Island, B
- offshore Paramushir
Island.
Soloviev & Ginsburg: Formation of submarine gas hydrates
Fig. 10. Lithological and hydrogeochemical control of gas hy-
drate locations, Site 491 DSDP, offshore Mexico (Initial Re-
ports...,
1982). A - subbottom depth, m; B - geological age; C -
particle size distribution,
% (1
- sand, 2 - silt,
3
-
clay);
D
- gas
hydrate locations; E - chlorinity of pore water,
%o.
indicated by the seismic evidence of gas-charged sedi-
ments.
American Trench is a typical example. On seismic pro-
files,
the distinct landward-dipping reflectors (LDR) may
indicate the routes of squeezed gas-bearing fluids migra-
ting to the gas hydrate formation zones (Fig. 12). Wide-
spread bottom simulating reflectors (BSR) identified as
the base of gas hydrate-bearing sediments (Shipley et al.
1979) are indicative of gas hydrate formation also in-
volving upward migrating gas-bearing fluids.
Discussion and conclusions
Generally, the presented data support the assumption that
fluid infiltration is trie major hydrate-forming process in
the submarine environment. This may be observed at
different scales - from individual specimens to world-
wide localities.
1.
Comparatively permeable lithological horizons and
fractures are present in hydrate-bearing intervals of
ocean drilling holes. The structures of hydrate-con-
taining sediments also suggest that hydrates should
form by moving fluids and should be associated with
faults.
Gas hydrates fill fractures and large voids and
cement comparatively coarse-grained sediments.
2.
In regions with hydrates, it is possible to establish a
direct relationship or spatial link between hydrate oc-
currences and geological structures which may control
the movements of fluids towards the sea bottom.
These are faults, diapirs, mud volcanoes, crests of
anticlines, layered sequences and updipping reflectors
probably caused by bedding or fracturing.
3.
Hydrate occurrences are restricted to continental mar-
gins and to intra-continental and marginalseas. Sub-
marine seepages (of various origins) and pockmark
areas coincide with the same worldwide structures.
This is well illustrated by the map of gas hydrate
Other locations
Data on gas hydrates recovered by bottom sampling in
the Gulf of Mexico (Brooks et al. 1984) and offshore
North California (Brooks et al. 1991) also prove that gas
hydrate occurrences are associated with submarine fluid
seepage areas and, in particular, with mud volcanoes.
According to the deep-sea drilling data , the holes that
recovered hydrate-containing sediments (Legs 66, 67, 76,
84,
96, 112, 127, 131 and 146 DSDP-ODP) show a
certain lithological control of gas hydrates. Normally
their occurrences are associated with relatively porous
ash and silty sands (Fig. 10) as well as fracture zones
(Fig. 11).
All the areas where gas hydrates have been discovered
in the course of deep-sea drilling are characterized by
specific geological and hydrogeological settings in which
fluids are squeezed from the sediments. This may be
either by mere simple consolidation (e.g. Blake Outer
Ridge area) or by tectonic overpressure in subduction
zones (deep water trenches). The slope of the Middle Fig.
11.
Fractures in hydrate-containing site sections according
to paleomagnetic data, offshore Mexico (Initial Reports..,1982).
92 Bulletin of the Geological Society of Denmark
Fig. 12. Characteristic
seismic reflectors which are
presumed to be fluid-
conductors in hydrate-
bearing region, Middle
American Trench, offshore
Mexico (Initial Reports...,
1982,
1985). BSR - Bottom
Simulating Reflection; LDR
- Landward Dipping
Reflection; 486-492 - DSDP
Sites.
- 487 486
Crust
o{
the
ocean
-^Æ
10-L
Kttl
.^. "" , IK X.« X X
\LDR/ xx*
10 20Km
i .10
Km
locations in the oceans (see Fig. 1) and the map of
pockmark and seepage distribution (Hovland and Judd
1988:
Fig. 4.73).
Gas hydrates in the submarine environment may be
formed both from water-dissolved gas and from free gas.
The formation of gas hydrates in these contrasting sit-
uations is substantially different (Fig. 13). Water has an
important advantage over gas. Dissolved salts, concen-
trated as a result of the transformation of the water frac-
tion into hydrate, are removed by filtering the flow. In the
case of gas infiltration, salts remain in the reaction zone
and inhibit the process. Hydrate films forming at the
gas-water interface are also inhibitors. Thus, the kinetics
of hydrate formation from free gas are evidently charac-
terized by the fact that diffusion plays a significant role.
In the vicinity of an ascending flow of gas-saturated
water, the reaction zone seems to be located where gas
hydrates are forming from diffusing gas.
Natural gas hydrates derived from gas-saturated infil-
trating water are likely to be the most common. The most
significant hydrate-forming infiltration takes place under
the conditions in which pore water is released from sedi-
ments by geostatic pressure and tectonic overpressure. A
similar model has been recently developed by Hyndman
(1990) on the basis of seismic survey data (BSR). In this
model, gas hydrates form from pore fluids that are ex-
pelled from an accretionary wedge. Another cause of
water infiltration might be thermo-artesian pressure
forming under local heating from a magma chamber or a
cooling intrusion. Sediments heated in this way may
generate hydrocarbon gases.
Thus,
gas hydrates form mainly from fluids infiltrating
towards the sea floor through or from a thermobaric
hydrate stability zone. This zone appears to act as an
ocean-wide gas-geochemical barrier.
Dansk sammendrag
Det foreliggende arbejde beskriver forekomsten af gas-
hydrater og deres kemiske sammnensætning fundet i Sor-
tehavet, det Kaspiske Hav og i det Okhotske Hav.
Dannelsen af gashydrater er et almindeligt forekom-
mende geologisk fænomen i det submarine miljø i for-
bindelse med relativt dybt vand (500-2000 meter). Gas-
hydrater findes som regel på kontinentalskråninger og i
marginale eller intra-kontinentale have, ofte i forbindelse
med sedimentære bassiner med et relative tykt dække af
hurtigt akkumulerende unge sedimenter. Årsagen til dan-
nelsen af gashydrater er imidlertid kun dårlig kendt.
Gashydrater dannes almindeligvis udfra opløst gas i
formationsvandet, men laboratorieforsøg tyder på, at gas-
hydrater også kan dannes i forbindelse med fri gas. Gas-
Fig. 13. Gas hydrate
formation zones and fluid
migration in vicinity of
seabed seepages of gas-
saturated water (A) and free
gas (B). 1 - fluid-conducting
zone,
2 - main direction of
fluid flow, 3 - zone of
hydrate formation by
precipitation from filtering
water, 4 - zone in which
diffusion is sufficient to lead
to the formation of
concretionary hydrates, 5 -
zone of gas-undersaturated
water, 6 - direction of
diffusion flow of dissolved
gas,
7 - opposite direction
of water flow into the
hydrate formation zone.
•.•.•.•.-*/«V\ V\/
it?'.-
-.•.••y*'+ij-)(z A«""*-,/
Sea Bottom
**•>> • »r• Ait . . *\-
:i
© o I +/V" I ° •
\Z3 DQ EZ X'
::5
Soloviev & Ginsburg: Formation of submarine gas hydrates 93
hydrater findes overvejende i porøse og permeable over-
fladenære sedimenter, og de geologiske forhold tyder på,
at deres dannelse er betinget af infiltration af gasholdigt
formationsvand fra dybere liggende lag. Forekomsten af
gashydrater er således som regel forbundet med andre
gasrelaterede geologiske fænomener så som submarine
gasudslip, mudder vulkaner, pockmarks og gasmættede
sedimenter. Disse overfladenære fænomener kan ofte re-
lateres til dybere liggende geologiske strukturer, som
f.eks. forkastninger og diapirer, som kan være den ud-
løsende årsag til migration af fri gas eller gasmættet
formationsvand til de overfladenære sedimenter og der-
med dannelsen af gashydrat.
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94 Bulletin of the Geological Society of Denmark
